1. Field of Invention
Winding many turns of a fine metallic fiber tow wound about a rectangle of sheet metal produces an electrode of large surface area which is well suited for electrolytic water purification and other electrolytic processes. An electrode of this description can also serve as a battery plaque.
2. Description of Prior Art
In process electrochemistry, increasing electrode surface area improves the kinetics of the electrochemical process at low reactant concentration. Increased surface area also decreases the true current density at the surface in proportion, allowing the cell to operate at lower voltage and increasing the service life of the electrode. In batteries, increased surface area of the electrode plaques provides improved contact with the active material, decreasing overvoltage and improving energy conversion efficiency. In practice, large surface area process electrodes and battery plaques are very similar and their design is governed by much the same criteria, allowing technology to be usefully transferred between the two fields.
In U.S. Pat. No. 3,895,960 Brown et al. provided an electrode plaque made by compressing and diffusion bonding iron fibers, attaching a current collector by mechanical means or by welding, and plating the entire assembly with nickel to provide the needed electrocatalytic surface properties. In Brown's Example 1, iron fibers with length:diameter ratio of about 1,900 were used to produce an electrode plaque with 95% porosity, 0.025 inch thickness, and specific area 100 cm2/cm3. In U.S. Pat. No. 3,835,514 Pollock provided a similar electrode plaque with L:D of 800 to 8000:1, porosity of 70 to 97% and a diffusion bonded bus connector.
In U.S. Pat. No. 4,331,523 Kawasaki described electrodes suitable for water electrolysis comprising a perforate current collector, preferably titanium expanded mesh or titanium perforated plate coated with platinum group metals, with a “fibrous assembly” pressed against it to provide large surface area. He noted that the fibrous assembly could comprise a diffusion bonded “web” of titanium fibers coated with platinum groups metals. (Here and throughout, we use the term “platinum group metals” to mean the metallic elements Ru, Rh, Pd, Os, Ir and Pt and also their oxides.) Kawasaki did not specify L:D, porosity or specific area of the “fibrous assembly” in his electrodes, but his examples suggest values similar to those taught in U.S. Pat. Nos. 3,895,960 and 5,294,319.
In U.S. Pat. No. 4,708,888 Mitchell et al. described an electrode produced by applying an electrocatalytic coating to a fine titanium expanded mesh, then spot welding or metallurgically bonding current distributor members (also coated Ti) to the coated mesh.
In U.S. Pat. No. 5,294,319 Kaczur et al. combined and improved upon the elements of U.S. Pat. Nos. 3,895,960 and 4,331,523 to provide an electrode comprising a mat of randomly oriented cut titanium fibers of at least two distinct diameters with length:diameter greater than 1000:1 coated with platinum group metals and spot welded to a similarly coated titanium plate.
In U.S. Pat. No. 5,439,577 Weres and Hoffmann described but did not make an electrochemical cell wherein the anode comprised a porous body of particles or fibers of titanium sintered together and coated as described by them in U.S. Pat. No. 5,419,824. The cell comprised a series of such porous anodes in alternation with porous cathodes with porous separators between them, with the electrolyte forced to flow through this stack. In U.S. Pat. No. 5,439,577 they also described but did not make an electrochemical cell comprising (1) “an appropriate cylindrical form” with (2) a layer of coated Ti-fiber tow wound around it, followed by (3) “a nonconductive separator,” followed by (4) a “permeable cathode,” followed by (5) another noncounductive separator, and so in repetition of (2) to (5).
The electrode provided by Coin et al. in U.S. Pat. No. 5,783,050 comprises multiple layers of Ti-expanded mesh wound on a Ti-plate with an electrocatalytic coating applied to the assembly.
The electrode provided by Morin in U.S. Pat. No. 4,680,100 comprises a thick tow of thousands of very fine nonmetallic fibers coated with a thin layer of metal and wound on a nonconductive plastic substrate member with electrical connectors attached to the ends of the tow using solder.